Operators of mobile systems, such as universal mobile telecommunications systems (UMTS) and its offspring including LTE (long term evolution) and LTE-advanced, continue to rely on advanced features to improve the performance of their radio access networks (RANs). These RANs typically utilize multiple-access technologies capable of supporting communications with multiple users using radio frequency (RF) signals and sharing available system resources such as bandwidth and transmit power.
Recently, LTE systems have begun to extend their operation into unlicensed frequency bands such as the 5 GHz band, which is currently primarily used by WiFi systems conforming to the IEEE 802.11 specification. A technical specification being developed for the use of LTE technology in unlicensed bands is referred to as LTE in Unlicensed (LTE-U). Because of the additional frequency resources that are made available by the use of an unlicensed frequency band, it is possible to assign different, non-overlapping channels to different cells, simultaneously allowing system capacity improvements and reductions in interference.
An important principle that is to be observed when operating LTE in an unlicensed band is to ensure that LTE-U co-exists with current technologies such as Wi-Fi on a fair basis that allows both technologies to use channels in that band. More particularly, one priority is that LTE-U should not behave more aggressively toward an access point using the competing technology (e.g., Wi-Fi) than two access points using the competing technology would behave toward one another. That is, LTE-U should not degrade the performance of the competing technology any more than would two interfering devices that both use the competing technology.
In general, coexistence mechanisms begin by selecting a channel in the unlicensed band that is currently not being used by the competing technology in order to avoid interference. A channel selection algorithm monitors the operating channel on an on-going basis and will change to a more suitable channel if needed. If no unused channel is available, a Carrier Sensing Adaptive Transmission (CSAT) algorithm is used to apply time-division multiplexing based access (TDMA) techniques to LTE-U cells, based on long-term carrier sensing of co-channel activities of the competing technologies. In this way the two technologies can share the channel fairly. In particular, CSAT defines a time cycle and the LTE-U cell transmits in a fraction of the cycle and gates off for the remainder of the cycle. The duty cycle of transmission vs. gating off is dictated by the sensed medium activity of the competing technology.
One issue that needs to be addressed when CSAT is employed as a coexistence mechanism concerns how the aforementioned co-existence principles can be observed while optimizing system performance.